The present invention relates to medical fluid pressure transducers, and, more particularly, to fluid pressure transducers for invasive blood pressure measurements having a reusable component and a one time use disposable component.
Since the 1970s, physiological blood pressure monitoring became widely employed for diagnosis and treatment of patients experiencing hemodynamic instability during surgery and in other forms of acute illness. An arterial cannula, central venous catheter, or pulmonary artery catheter is inserted into a blood vessel using a Seldinger percutaneous puncture technique, the puncture wound is dilated, then the catheter is inserted into the vessel, the catheter is attached to a saline-filled pressure monitoring line, blood pressure transducer, pressurized fluid supply, and sterile fluid flushing device, and finally the transducer's electrical interface cable is attached to an electronic blood pressure amplifier and display monitor. Once calibrated, such systems give accurate and up to date readouts of the constantly changing blood pressure levels at the tip of the catheter within the cardiovascular system.
Similarly, catheters and techniques have been developed for direct insertion of fluid filled catheters into the brain for the monitoring of increased intra-cranial pressures resulting from acute brain injury, and for insertion into the uterus during childbirth to monitor the strength and character of contractions through changes in the amniotic fluid pressure. Many of these same prior-art transducer systems have been and continue to be employed for this entire range of measuring applications, plus other physiological monitoring or biological fluid pressure measurement applications within living bodies.
A typical early prior art device includes a removable (single use disposable) dome with an inlet and an outlet port for flushing and filling of the transducer assembly with sterile isotonic saline solution. The dome is made of a clear molded plastic material such as polycarbonate so that air bubbles in the flushing fluid could be observed and removed. A flushing device, such as ones described in U.S. Pat. Nos. 4,291,702 to Cole or 3,675,891 to Reynolds is typically affixed to the dome's side port for the purpose of providing a continuous flow of saline through the dome and into the catheter. A "fast flush" valve on the flush device may be activated to temporarily select a higher flow rate for filling, de-bubbling, or clearing of blood in the dome and/or catheter.
The early prior art blood pressure transducers were made with a metal diaphragm forming a pressure sensitive area on an external surface of the transducer housing. The diaphragm was coupled internally to a mechanical push rod linkage assembly and a strain-sensing device, such as an unbonded wire strain gauge constructed in a Wheatstone Bridge configuration. The pressure sensitive area of the metal diaphragm is now typically isolated from the sterile saline being flushed into the catheter by a thin polycarbonate or nitrile rubber diaphragm on the mating surface of a single use dome. Such disposable domes are typically supplied sterile and discarded after a single use to avoid a biological contamination risk to the patient. The mating reusable transducer portion, which is not in direct contact with the patients' blood, is frequently wiped down with alcohol or placed in a chemical sterilant after each use and then reassembled to a new, sterile dome for subsequent uses. In the late '70s, several physiological pressure transducers systems were developed using semiconductor strain sensors, but still employing a mechanical linkage and a metal diaphragm, for example the Statham P50 and the Bentley M800. The strain sensing element is a silicon beam which is bonded to the transducer body in such a way that strain is applied to the beam when the diaphragm is flexed. In these designs, the Wheatstone bridge was ion-implanted directly into the silicon beam and the output signals were calibrated using discrete resistors located in the transducer's electrical interface connector. In other respects, these "transitional" art transducers were typically connected to catheter/manometer fluid systems in the same manner as described above.
In the early '80s, non-reusable (disposable) transducers were developed using improved semiconductor strain gauges made from a silicon chip containing an etched diaphragm used in combination with laser-trimmed thick film resistor network for temperature and span compensation of the sensor chip, as is fully described in U.S. Pat. Nos. 4,576,181, and 4,291,702, 4,557,269, 4,683,894, 4,679,567, 5,042,495, 4,776,343, 5,097,841. Further, the development of thin film "on-chip" compensation methods at Motorola (see U.S. Pat. No. 4,465,075) allowed the development of even smaller, simpler disposable transducer designs as are more fully described for example in U.S. Pat. Nos. 4,539,998, 4,679,567, and 4,825,876. Importantly, all of these disposable transducer designs except those of Cole and Kodama appear to have abandoned the mechanical linkage in favor of a hydraulic pressure coupling medium comprised of a silicone elastomer, or "silicone gel", for example as cited in U.S. Pat. No. 4,529,789. These elastomers, which had become common in the semiconductor industry for protecting chips from ambient fluids and vapors, are used to form a good electrical barrier between the chip and the saline solution, while imparting greater mechanical ruggedness and over-pressure characteristics to the sensor, plus transmission of the hydrodynamic signal. In medical use, the gel is juxtapositioned between the catheter flush solution and the transducer chip, thus conveying the hydraulic pressure signal directly to the chip's integral sensing diaphragm while isolating it electrically from the conductive and corrosive effects of the saline solution. The entire transducer assembly, including the chip are typically sold to be discarded after a single use, since the internal components can not be adequately cleaned for resterilization or reuse.
Disposable transducer designs employing semiconductor strain gauge sensors and gel coupling media as just described are desirable because they provide a relatively straight fluid channel which is easy to fill with sterile saline without turbulence or accumulation of bubbles. Further, they do not require twist-on attachment of a separate disposable dome as the prior art re-usable designs do, and they are highly rugged and accurate due to the gel pressure transmission media and silicon chip micromachined sensor structure. However, manufacturing costs remain high. The single highest cost component is the pre-calibrated semiconductor chip and associated wiring, which typically must be discarded after a single use.
With the objective of further reducing medical costs and medical wastes, accordingly, there is a need for a re-usable physiological pressure transducer which employs an inexpensive semiconductor strain gauge sensor which has been produced by current high volume silicon micro-machining and chip carrier production techniques. Wallace (U.S. Pat. No. 4,610,256) and Frank (International Application PCT/US85/01957) disclose pressure transducers employing thick-film trimmed silicon strain gauge sensors attached to a transducer body filled with silicone oil. The exterior of the transducer body carries a pressure sensitive area covered by a flexible diaphragm which communicates hydraulic signals to the chip sensor through the silicone oil-filled body and through a hole filled with a pressure transmissive fluid which is respectively gel or oil to an opposing exterior surface. In these designs, the silicon chip sensor and compensation circuitry is carried by an exterior opposed surface of the transducer body. In both of these examples, the mating disposable dome contains a flexible sterile isolating diaphragm according to the disposable dome prior art which is intended for one time use. Adams, et al. (U.S. Pat. No. 4,686,764) discloses a gel-filled pressure transducer body containing a thin film-trimmed chip sensor. The silicon chip sensor is located inside the body and the pressure sensitive area on the exterior of the transducer consists of a flexible polymer membrane such as polyamide which transmits the hydraulic pressure signal through the gel and thus directly to the sensor without need for a coupling channel. Frank (U.S. Pat. No. 4,920,972) discloses a blood pressure transducer comprising a gel-filled body with a chip sensor again located on the outside of a body and hydraulically coupled to the diaphragm through a tapered hole filled with gel. The transducer diaphragm covering the pressure sensitive area on the opposing side of the body is a flexible material such as silicone rubber. A disposable dome of the prior art type using a flexible interface membrane is used to isolate the sterile saline from the transducer.
In spite of these improvements, nonetheless, the prior art transducers still suffer from certain drawbacks. The disposable transducers remain expensive to produce because of the high cost of throwing away the micromachined chip and wiring. The diaphragm of the earlier prior art re-usable transducers disclosed by Wallace and Frank can be easily punctured resulting in leakage of the pressure transmitting medium and failure of the transducer. While the re-usable transducers as later disclosed by Adams and Frank offer a significant improvement in mechanical ruggedness because of the use of silicone gel as the hydraulic coupling medium, the fluid path inside the dome is tortuous and still more difficult to setup, fill and de-bubble because the membrane-type dome is large in relation to the diameter of the inlet and outlet ports, and it must be attached and filled with saline prior to use. In practice, small bubbles often attach themselves in the sharp corners adjacent to the edges of the diaphragm, thus reducing the dynamic response of the transduced pressure signal.
Therefore, it would be highly desirable to develop a disposable dome for a reusable transducer application employing a fluid path without sharp corners, for instance, in the vicinity of the diaphragm, where bubbles are easily entrapped in current designs. It would also be highly desirable to develop a disposable dome structure for a re-usable transducer which has a more-or-less straight through fluid filling path to minimize the time and difficulty required to clear the system of air bubbles. It would also be desirable to reduce manufacturing cost and complexity of the medical reusable transducer design by placing the sensor chip directly inside the transducer body in communication with pressure hydraulic transmitting medium. And finally, in the prior art devices of the re-usable type, there is no physical barrier to prevent the medical practitioner from touching the non-sterile parts of the transducer while attending to the patient. It would be highly desirable to develop a means for isolating the non-sterile (reusable) parts from the sterilized parts normally manipulated by the medical practitioner during blood drawing and readjustment and calibration of the catheter and monitoring system components. These and other objects and advantages of the present invention will be apparent from the attached drawings and the description of the preferred embodiments which follow.